Global warming: What happens if the sun loses its spots?

The notion that the sun could be heading for a grand minimum hit the headlines two years ago, when three research teams using independent measures suggested that the next sunspot cycle's activity could be substantially lower than the current cycle's.

One sign: A fairly steady decline in the strength of the spots' magnetic fields over a 13-year-period. If the trend is to continue, scientists said, they anticipated a spotless sun by around 2022.

"We still see a decrease in the sunspot magnetic fields," says Matt Penn, a researcher with the National Solar Observatory in Tucson, who took part in the study. "The results are consistent with what we presented in 2011. It seems the trend is continuing along that line" leading to a cut-off in sunspot production.

More recently, research has suggested that the strength of the suns' magnetic field during one solar minimum – when the field is at its strongest – is a harbinger of the size of the peak for the next sunspot maximum. Over the past three sunspot cycles, those fields at solar minimum have been getting weaker, with the weakest appearing during the most recent minimum.

Given these trends, "I don't see how we're going to get fields any stronger this time around than they were" prior to the current solar maximum, known as cycle 24, says David Hathaway, a solar physicist at NASA's Marshall Space Flight Center in Hunstville, Ala. "It looks like cycle 25 might be smaller yet."

"My suspicion: If you think this cycle's bad, wait for then next one," he says.

Meanwhile, over the past decade climate scientists have evolved a better understanding of how – between the valleys and peaks of the sunspot cycle – a tiny increase in the energy the sun radiates toward Earth can affect climate.

Two mechanisms have emerged that can have a measurable effect, especially regional climate. The center of action for both is the tropical Pacific, and to a lesser extent, the North Atlantic, Dr. Meehl explains.